Impacts of heatwaves and toxic algal blooms on the physiological performance and future aquaculture of the oysters Ostrea edulis and Magallana (Crassostrea) gigas

This thesis aims to increase our understanding of the vulnerabilities of bivalves under future climate change (CC) scenarios and identify adaptations aquaculturists, policymakers, and conservation managers should employ for optimal success. After the General Introduction, Chapter Two presents a systematic review of the state of research on the impact of multiple CC stressors on bivalves. Future studies should include less-studied, economically important bivalve species, early life stages, understudied vital responses (e.g., reproduction and behaviour), and relevant CC stressors combinations (e.g., warming and harmful algal blooms). Chapter three suggests that Ostrea edulis is vulnerable to the increased frequency of heatwaves. The upper thermal optimum of O. edulis is 24°C, while the lethal temperature is above 33°C. Sustained periods between these temperatures reduced body condition and increased mortality. Chapter four investigates the combined effects of a heatwave and the presence of the dinoflagellate Prorocentrum lima, a species that produces diarrhetic shellfish toxin-associated algal blooms (DSTB). P. lima significantly dampened the typical metabolic response of Magallana gigas to warming. Oysters exposed to this toxic alga displayed a subdued reaction to warming, raising concerns about their ability to adapt effectively to sudden temperature shifts. P. lima and heatwave-exposed oysters accumulated more toxins than those exposed to P. lima alone, suggesting more toxic shellfish as heatwaves become more frequent. Chapter five investigates whether bivalves can be used as biosensors for DSTB. The behaviour of M. gigas changed after DSTB exposure, spending less time wide open. Bivalve aquaculturists and restoration practitioners are recommended to choose subtidal and lower intertidal habitats. Monitoring blooms and shellfish meat should be more frequent and efficient as warming may induce higher toxin accumulation. Valve sensors should be deployed in sites where there is a regular bloom occurrence for the advancement of a system that uses bivalves as algal bloom biosensors.